Traditional methods of dealing with municipal solid waste generally included one of two alternatives: a) is the waste was burned; or b) the waste was buried. However, landfills and incineration are no longer preferred or even viable solutions due to diminishing area available, environmental concerns and cost to develop/maintain. A landfill capable of servicing a mid size municipality may run into the hundreds of millions of dollars and an approval process may take ten or fifteen years. Once in operation, the full life of a landfill may be shortened due to environmental concern.
Incineration of waste creates greenhouse gases which are being targeted to be reduced, thus processes that produce them are being considered to be unacceptable. In the United States, the number of operating landfills has diminished significantly over the last several decades. Consequently, alternative ways of dealing with municipal waste must be sought.
It is known for biomass to be treated with Pyrolysis or similar heating methods that limit an amount of oxygen available to the waste and thereby eliminate combustion, such as, for example the system described in U.S. Pat. No. 7,293,511. In some known processes an oil, is generated which may be burned in a relatively clean fashion. Conditions for producing pyrolysis oil will typically include virtually no oxygen. Pyrolysis oil or other thermo-chemically-derived biomass liquids can be used directly as fuel, or sometimes as a platform to produce chemicals and materials.
Currently known Pyrolysis machines include a heat chamber and a heat source located around the periphery of the heating chamber. Although these units provide a proof of concept to a basic science of conversion of biomass to a usable product, their effectiveness has only been applicable for relatively homogeneous biomass, such as byproducts of a paper mill, and only for modest amounts of biomass conversion. In addition, the efficiency of these known types of pyrolysis units is generally less than desirable.
Fast pyrolysis includes thermal decomposition of a biomass fuel at moderate temperatures with a relatively high heat transfer rate to the biomass particles and a short hot vapor residence time in a reaction zone. Several reactor configurations have been shown to assure such conditions and to achieve yields of a burnable liquid byproduct. Designs of such pyrolysis apparatus may include bubbling fluid beds, circulating and transported beds, cyclonic reactors, and ablative reactors.
Biomass pyrolysis research has been directed to vortex (cyclonic) and fluidized bed reactors for processing biomass via pyrolysis. The fluidized bed reactor of the Thermo chemical Users Facility at the National Renewable Energy Laboratory is a 1.8 m high cylindrical vessel of 20 cm diameter in the lower (fluidization) zone, expanded to 36 cm diameter in the freeboard section. It is equipped with a perforated gas distribution plate and an internal cyclone to retain entrained bed media (typically sand). The reactor is heated electrically and can operate at temperatures up to 700° C. at a throughput of 15-20 kg/h of biomass.
Some experimental pyrolysis technologies have been demonstrated which utilize circulating fluidized bed plants. However, each of the fluidized bed plant models requires homogeneous biomass and can only scale to a size too small to function effectively for a municipality. Generally, the previously known pyrolysis machines expose biomass to 550° C., in an oxygen-deprived environment and do not provide an adequate amount of heat transfer area for a uniform amount of heat to consistently treat municipal waste.
Accordingly, new apparatus and methods are needed to efficiently treat municipal waste on a scale suitable for a small city or waste processing area.